During the attack the pair took selfies of themselves and even posted a picture on social media, taken in the police van after the incident.

The girls, who are now both 15, were in tears in the dock after they were convicted at Leeds Crown Court.

Prosecutors said afterwards: "In our society it is hard to imagine that two girls of such a young age could be capable of such violence."

Miss Wrightson was found dead in her blood-spattered living room in Hartlepool, County Durham, in December 2014.

Police said that of her 100 injuries, 25 were made by weapons which included a coffee table, a computer printer, a wooden stick laced with screws, a television set, a shovel, ornaments, a picture frame and a kettle.

The girls' accounts of what happened differed, but the jury heard that Miss Wrightson, who was 5ft 4ins and weighed six-and-a-half stone, was assaulted in 12 separate locations around the room.

While at the house the younger girl made a phone call over Facebook to a friend who heard her say: "Go on (older girl). Smash her head in. Bray her. F****** kill her," as another laughed in the background.

The court heard that the defendants left the house for "time out" at around 11pm, during which, covered in blood, they went to see a friend.

The pair returned to the scene at around 2am and stayed for a further two hours, before calling the police to take them home.

The court heard they left Miss Wrightson in an "undignified manner", naked from the waist down on her sofa.

Officers who collected the girls described them as laughing and joking and in "high spirits".

An earlier selfie posted to Snapchat showed the defendants smiling with Miss Wrightson pictured in the background shortly before her death, with further selfies showing the girls drinking cider from a bottle.

The picture taken in the police vehicle was posted to the social media site Snapchat with the message: "Me and (older girl) in the back, on the bizzie van again."

The jury was told that shards of glass and small pieces of gravel or grit were strewn over and around Miss Wrightson's genitals and ash from burnt paper had been put into her ear.

Her living room was described as "akin to a bomb site".

The trial heard that the girls had visited Miss Wrightson, an alcoholic known as "Alco Ange", on a number of occasions as she would buy them alcohol and cigarettes.

On the evening of the murder they let themselves into her home and asked Miss Wrightson to go to the shop for them.

Both had been drinking before they arrived and the older girl told the court she had taken prescription drugs earlier in the day.

The jury of eight women and four men deliberated for just over three hours before returning the guilty verdicts.

The girls were in tears straight after the verdict and the judge, Mr Justice Globe, asked for them to leave the court.

They later returned to court to be told they will be sentenced on Thursday.

Gerry Wareham of the Crown Prosecution Service said: "In our society it is hard to imagine that two girls of such a young age could be capable of such violence.

"The attack that the girls committed against Angela Wrightson was brutal and sustained. One can only imagine the fear and distress that she must have felt in the final hours of her life."